const In : set set prop term iIn = In infix iIn 2000 2000 const famunion : set (set set) set axiom famunionE: !x:set.!f:set set.!y:set.y iIn famunion x f -> ?z:set.z iIn x & y iIn f z claim !x:set.!f:set set.!y:set.y iIn famunion x f -> !P:prop.(!z:set.z iIn x -> y iIn f z -> P) -> P